1. Field of the Invention
The invention is generally related to fasteners for structural steel framing and more specifically to a bolt configuration for high-strength tension control fasteners.
2. Description of the Related Art
Structural steel framing is used to provide vertical and horizontal stability in numerous civil construction applications requiring high-strength fasteners of large steel members, including by way of example, large buildings, industrial frameworks, bridges and towers.
Tension control fasteners have become commonly used in the field of structural steel framing, particularly for the connection of structural steel framing members. A significant advantage of tension control fasteners is that they can be applied with an appropriately designed installation tool to a predetermined torque relatively quickly.
A prior art tension control fastener set includes a bolt and a nut with washers as required by the particular application. An exemplary tension control bolt incorporates a breakaway section at the end of the threaded portion of the bolt, with a groove intermediate the threaded portion and the breakaway section. The breakaway section is designed to break off, at the reduced-diameter groove, when a tension above a design level is induced in the bolt during the installation. An installation tool holds the bolt, grips the breakaway section with a conforming wrench chuck and tightens the nut until the breakaway section twists off at the groove.
Prior art tension control fasteners include bolts having conventional hexagonal heads providing surfaces for interface with wrenches and bolts having rivet or rounded heads without wrenching surfaces. The first type of bolt is referred to herein as hexagonal head bolts and the second type is referred to herein as rivet head bolts.
Because of structural and safety concerns a number of organizations have developed structural steel framing industry standards and specifications for the manufacture, application and installation of fasteners for connecting structural steel framing members. Such organizations include the American National Standards Institute (“ANSI”), the ASTM International (“ASTM”), the Research Counsel on Structural Connections (“RCSC”) and the American Institute of Steel Construction (“AISC”).
The usual material specification for conventional high strength hexagonal head bolts in steel-to-steel structural connections is ASTM A325. ASTM A490 is specified when higher strength is required. ASTM specification F1852 is a standard developed specifically for tension control bolts previously manufactured to A325 specifications. ASTM F1852 allows for the use of hexagonal head bolts, rivet head bolts and alternate design heads. Alternate design heads may be allowed under such specifications provided that they have a bearing surface and head height equal to or larger than required for heavy hex structural bolts in ANSI B18.2.1.
The connection of two members or plies, each possessing a hole, is inherent in the use of fasteners for structural steel members. Such holes may be of different shapes and sizes. Descriptions of such holes include: (1) a standard hole, (2) an oversized hole, (3) a short-slot hole or (4) a long-slot hole. Holes may be classified by size in relationship to the appropriate bolt as follows:                a standard hole=db plus {fraction (1/16)} inch (˜1.6 mm);        an oversized hole=db plus up to {fraction (5/16)} inch (˜8.0 mm);        a short-slotted hole=(db plus up to {fraction (1/16)} inch [˜1.6 mm]) by (db plus up to ⅜ inch [˜9.6 mm]); and        a long-slotted hole=(db plus up to {fraction (1/16)} inch [˜1.6 mm]) by (2.5 times the db).        
As used herein, “db” means bolt diameter.
The stress nature of the connection establishes minimum requirements for size of the bolt shank. Holes in the members to be connected must be large enough to accommodate the minimum required bolt diameter.
The size and type of hole contacting the head determines the requirement for a washer under the head of the bolt. Generally, hexagonal head bolts and rivet head bolts may be used without washers intermediate the bolt head and the member when the bolt head is adjacent a ply having a standard hole, but require washers when used with other types of holes. Bolts may be used without washers intermediate the bolt head and the ply in a standard hole. Generally, washers are required in oversized holes, short-slotted holes and long-slotted holes. However, alternate design head bolts may be approved for use without washers. Rivet head bolts are used in standard, oversize and short-slot holes without washers intermediate the bolt head and the ply if the rivet head bolt has a diameter equivalent to the diameter of the structural steel framing industry standard washer that would otherwise be applied.
Examples of the prior art follow:
U.S. Pat. No. 2,928,302 issued to Owen et al. on Mar. 15, 1960, discloses a means for achieving a predetermined extent of loading when tightening up nuts on bolts and studs, and provides a bolt or stud adapted to achieve this result when employing an appropriate tool for tightening up the nut, the bolt or stud having a tool anchoring part which shears off when a predetermined extent of reaction torque has been applied thereto in tightening up the nut.
U.S. Pat. No. 4,177,352 issued to Dunbar et al. on Dec. 4, 1979 discloses a shear bolt assembly wherein an independent shear component is fastened into the end of the bolt shank in order to provide torsional relief from lateral stress.
U.S. Pat. No. 4,290,337 issued to Kuwata et al. on Sep. 22, 1981 discloses a fastener set comprising a bolt body, and a nut and washer which are fitted onto the bolt body, wherein the bolt body has a notched portion which can be broken off by a predetermined torsional force. A lubricant is disclosed to make the torsional force relatively uniform in a variety of atmospheric conditions.
U.S. Pat. No. 4,917,555 issued to Taubert on Apr. 17, 1990 discloses a joining element possessing parallel, concentric rings with a diameter greater than the fastening threads, to provide improved elasticity in the joining element when tensionally employed. Such patent does not address tension control bolts for structural steel framing.
U.S. Pat. No. 6,264,414 B1 issued to Hartmann et al. on Jul. 24, 2001 discloses a fastener possessing an elongation portion, a threaded portion and a fitting portion along the shank, with the threaded portion and the fitting portion having larger cross-sectional areas than the elongation portion to provide greater elasticity to the fastener when stressed. Such patent does not address tension control bolts for structural steel framing.
U.S. Pat. No. 6,343,904 B1 issued to Wang on Feb. 5, 2002 discloses a rivet-head tension bolt with anti-skid projections extending radially from the bearing surfaces of both the head and accompanying washer.
The thickness of the work pieces to be connected, a distance called the grip, determines the length of bolt needed to accommodate the size of the nut and washer. In applications requiring the use of a washer, the bolt length must be increased by the thickness of the washer to accommodate for the requirement of the washer. In many instances such additional length requires a longer, more expensive bolt. Additionally, industry specifications typically require that the hexagonal head of a hexagonal head bolt be equivalent in size to the nut required for the same bolt. This, in effect, mandates the cost of providing two fully effective wrenching surfaces when only one such surface is normally required for installation of the fastener set. Additionally, the required combined height of the hexagonal head and a washer is a disadvantage in instances where there is little working space. As the construction industry has recognized the foregoing and other disadvantages of conventional hexagonal head bolts, the use of oversize rivet head bolts has become increasingly common.
The bearing surface of rivet head bolts may eliminate the requirement to use a washer on the head end of a fastener set in many instances. However, prior art rivet head bolts suffer the disadvantage that they do not allow wrenching of the bolt head. Removal of rivet head bolts after installation may require torching or cutting the old bolt out of the hole because there is no wrenching surface on the rivet head. After a passage of time, removal of installed bolts is even more difficult due to paint, insulation, oxidation and other factors that increase resistance to torque. Also, every grade, diameter, length and lot number of bolts used for structural steel must be tested prior to installation. Removal of a rivet head bolt after such testing is required. Such removal may require cutting the bolt, as torches cannot be used adjacent test equipment.
Nicked or damaged threads may cause a rivet head bolt to roll during installation. Lack of a wrenching head renders completion of installation or removal of the partially-attached bolt difficult as there is no gripping surface on the rivet head.
Additionally, it is advantageous to provide a wrenching surface to install bolts when conventional tension control tooling cannot be used due to clearance.
It is also advantageous to provide a wrenching surface to re-tighten, referred to as touching up, when installation of nearby bolts results in the bolt loosening up.
An alternative method of applying required torque allows prescribed nut tightening with the bolt head prevented from rotating. A wrenching surface allows the nut to be held in place while torque is applied to the nut.
It would be an improvement to the prior art to provide a bolt head having the advantages of rivet head bolts while providing wrenching surfaces to allow an alternative means to install bolts when clearances do not allow the use of conventional tension control tooling.